1. Field of the Invention
The present invention relates in general to disk storage systems and more particularly, to a method and apparatus for detecting defects on a disk in a disk drive assembly.
2. Description of the Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is recorded on concentric tracks on either surface of one or more magnetic recording disks. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms which are rotated by a voice coil motor. The voice coil motor is excited with a current to rotate the actuator and move the heads. The read/write heads must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information. The read/write heads read recorded information from the surface of the disk by sensing the magnetic transitions emanating from the surface of the disk.
The ability to store and read back information from the disk in the form of magnetic transitions may be impaired by the presence of disk defects. Disk defects can manifest themselves in several ways. One way is simply as noise associated with the random nature of the disk surface. Often, this noise can be easily separated from the ambient noise in the readback channel by measuring the noise from the preamplifier with the head flying over the disk and then repeating the measurement with the head unloaded from the disk.
More troublesome are flaws or defects in the disk surface that manifest themselves as missing bits or as extra bits of data (i.e., as single bit errors). Missing bits are reductions in the amplitude of the envelope of the signal, usually over a small number of bits (e.g., 1-4 bits) such that the amplitude falls below the channeldetector clipping level. The number of missing bits observed depends on the setting of the clipping level in the read-back channel.
One common technique for avoiding the problem of missing or extra bits is to perform a surface analysis of the disk. This surface analysis yields an error map of the disk surface. The error map can then be referred to as a means of avoiding localized disk flaws. Surface analysis consists of scanning all of the data tracks on the disk for defects. The disk drive system typically utilizes an error correction code to attempt to correct the disk defects. The located defects which are uncorrectable are placed on a defect list which is later stored in random access memory (RAM) during the burn-in process. The defect list will then be made available to disk formatter programs, which write sector headers that identify defective sectors so that these sectors remain unused.
Disk defects, if left uncorrected, have a tendency to increase in size and thus affect adjacent bits and/or data sectors. Although conventional systems have generally been capable of disk defect detection, they lack the means for ascertaining the number, length and/or size of defects that exist on the disk. As a result, users of such conventional systems are unable to take precautionary measures in avoiding use of certain data sectors that are at risk of being affected by existing disk defects which have a tendency to increase in size.
Accordingly, there is a need in the technology for a method and apparatus for a method and apparatus for detecting the number and sizes of defects on a disk in a disk drive assembly. Such a method and apparatus facilitates improved defect detection and disk defect prediction, so that improved production yield may be achieved.